Remains of unidentified victims killed in the 9/11 attacks on the World Trade Center in New York have been returned to the site today in a ceremonial procession, despite criticism from some family members of those who died in the terror attacks.

A casket draped with a US flag during the ceremonial transfer of the 9/11 unidentified remains to the World Trade Center site. Credit: Reuters

The remains were moved to an underground repository at Ground Zero, the same building as the National September 11 Memorial Museum, which critics says could be prone to flooding.

Relatives of some victims wore black bands over their mouths at the site in protest.

Family members of victims of the 9/11 attacks protest against the decision to transfer the 9/11 unidentified remains to Ground Zero. Credit: Reuters

"Don't put them in the basement," said Rosemary Cain, who lost her firefighter son in the attacks, NBC News reported. "Give them respect so 3,000 souls can rest in peace."

The repository is sealed off from exhibition areas by a wall and will only be accessible to the medical examiner's staff and family members of the victims, who will be able to visit the space even when the museum is closed, the city has said.

A large piece of airplane debris found close to the World Trade Center site in New York is believed to be landing gear from one of the planes which struck the towers in 2001, according to NBC News.

The part bears a "Boeing" stamp, as well as a series of numbers. Credit: CBS

New York Police Department (NYPD) confirmed that the five-foot-long part was discovered by building surveyors between two buildings on the site where a mosque and community center has been proposed three streets away from Ground Zero.

The piece was found at the rear of 51 Park Place in New York. Credit: CBS

The part bears a "Boeing" stamp, followed by a series of numbers.

Police spokesman Paul Browne said the NYPD had secured the location and would treat it as a crime scene.

An aerial view of Manhatten shows the street's proximity to the World Trade Center site. Credit: New York Police Department (NYPD)